1. Technical Field
Aspects of this document relate generally to optical systems, and more specifically, to those that measure the ellipsometric parameters of matter within a low-loss optical cavity, including, in some implementations, within the evanescent wave generated by intra-cavity total reflection.
2. Background Art
Ellipsometry is a well-established technique for the nondestructive measurement of characteristics of material systems. The topic is well-described in a number of publications, one such publication being a publication by Collins, titled “Automatic Rotating Element Ellipsometers: Calibration. Operation and Real-Time Applications”, Rev. Sci. Instrum. 61(8) (1990). Practice of ellipsometry typically involves causing a spectroscopic beam of electromagnetic radiation, in an imposed, known, state of polarization, to interact with a material system at one or more angle(s) of incidence. Changes in the polarization state of the beam of electromagnetic radiation which occurs as a result of the interaction with the material system gives information about the structure and composition of the material system.
A typical goal in ellipsometry is to measure, for each angle of incidence and wavelength of a beam of electromagnetic radiation caused to interact with a material system, the ellipsometric parameters Ψ and δ (where Ψ is related to a change in a ratio of magnitudes of orthogonal components rp/rs in the beam of electromagnetic radiation, and δ is the phase shift introduced between the orthogonal components rp and rs, caused by interaction with the material system:ψ=|rp/rs|; andδ=(φp−φs).
The change of ratio rp/rs or the phase shift δ caused by the interaction of the electromagnetic radiation with the material system can be very small (e.g. δ can be a value such as 0.001°), so the accurate measurement of δ using a conventional system requires long data acquisition times for extensive signal averaging, which is sometimes impractical. Further, because of the limited time resolution inherent in conventional methods, fast events, where the value of δ changes quickly (in relation to the time resolution of the measuring system), cannot be measured by conventional systems and methods.
In cases for which the losses of electromagnetic radiation from interacting with such material systems are small (typically less than 1%), the effect from the interaction with such material system can be amplified by reflecting the radiation repeatedly so as to interact with the material system multiple times (usually at least 100 times). This has been achieved by placing the material system on the reflective surface of an optical cavity, as described in A.C.R. Pipino, “Ultrasensitive Surface Spectroscopy with a Miniature Optical Resonator” Phys. Rev. Lett. 83, 3093 (1999), M. A., Everest et al., “Hemoglobin Adsorption to Silica Monitored with Polarization-Dependent Evanescent-Wave Cavity Ring-Down Spectroscopy”, J. Phys. Chem. B 110, 19461 (2006), and U.S. Pat. Nos. 5,986,768 and 5,835,231, the disclosures of which are incorporated herein by this reference. In these cases, methods for the measurement of the losses for the p and s polarization states were described, which allows the determination of ψ=rp/rs with much greater sensitivity than single-pass techniques, and allows the determination of ψ on the microsecond timescale, However, these publications fail to provide methods or systems for utilizing multi-pass techniques to measure δ with increased sensitivity and time-resolution (for example, on the order of the microsecond timescale), Nor is it apparent from the known literature how to do so.
Resonator-enhanced optical inspection systems, and other optical systems, such as those described by U.S. Pat. Nos. 6,653,649, 6,700,840, 6,714,295, 6,717,707, 6,778,307, 7,330,277 B2, the specifications of which are incorporated herein by reference, provide improved resolution, surface detection and other performance improvements in traditional optical systems. However, measurement systems disclosed in the above-referenced U.S. patents are not able to measure the ellipsometric phase angle δ with a time-resolution of about 1 microsecond or less.
The publication Jacob et al., “Pulsed measurement of high-reflectivity mirror phase retardances”, Applied Optics, May 1994, vol. 33, No. 15, pp, 3175-3178, describes methods for the time-dependent measurement of the phase angle δ upon reflecting light from a high-reflectivity dielectric mirror, However, the measurement system described in this publication is for the characterization of high-reflectivity mirrors, not for the measurement of the ellipsometric parameters of materials. Highly-reflective dielectric mirrors are not ideal as substrates for ellipsometric measurements of materials because all but very careful sample preparation on the mirrors will significantly reduce the reflectivity of the mirrors.
In certain situations, where time resolution is important, no known conventional system cart accurately measure the ellipsometric parameter δ for some types of material. When the ellipsometric parameter δ is small, such as, for example, 0.001°, accurately measuring δ is often problematic or impossible for prior art apparatus and methods. Despite various ellipsometric techniques that have been developed in the past, apparatus and methods have not been developed which can accurately measure the ellipsometric parameter δ when δ is small (for example, smaller than about 0.001°) without the necessity for long data acquisition times to allow extensive signal averaging. Long data acquisition times and extensive signal averaging are not always feasible or possible. Further, in some situations the properties of the subject material change quickly. Signal averaging results in relatively poor time resolution. When the properties of the material change faster than the time resolution of an ellipsometric system, such system will not be able to accurately measure the time dependent change in the properties.
The relatively poor time resolution of prior art ellipsometry systems has limited the use of ellipsometry to measurements of the equilibrium or steady-state properties of materials.